This invention relates to a sewing machine needle, particularly for rapidly operating sewing machines which have high stitch output.
As a rule, the output of current industrial sewing machines is 5,000 stitches per minute; in some applications even 8,000 or more stitches per minute are reached.
During the sewing operation the sewing machine needle penetrates the workpiece (hereafter referred to as fabric) to produce a stitch hole. The time available for opening the stitch hole is the period from the moment the needle point touches the fabric to the moment when the needle eye passes the stitch hole. Such an opening period is approximately 0.5 ms for a stitching machine which operates with 5000 stitches per minute. The opening of the stitching hole occurs as a rapid, almost explosion-like lateral displacement of the fabric During such an occurrence, high frictional forces are generated and thus high penetrating forces as well as high operating power for the sewing operation are required.
Sewing with a very high number of stitches per minute involves the risk of mechanical and thermal damage to the fabric. Fibers or even fiber bundles (yarns) may be torn apart as stitch holes are being formed. Further, the high friction forces may heat the needle to temperatures which, within a few seconds, may cause the usual synthetic fibers to reach their melting point. Such fibers then are melted or fused in the stitching zone. A further consequence may be frequent ruptures of the sewing thread.
The fusing of laminates and layers as well as color or other components may lead to further disturbances during the sewing operation. The discussed problems may lead to a deteriorated seam quality and may adversely affect the strength of the manufactured apparel.
Further, needles for sewing machines, particularly those which operate at high machine speeds, are exposed to relatively large mechanical stresses. Increasing machine speeds cause dynamic problems to an increased extent. The masses rotating in the sewing machine may cause vibrations which affect particularly the sewing machine needles clamped at one end. The sewing machine needles furthermore have to withstand external forces such as the pull of the thread, lateral excursions upon contacting the fabric as well as forces derived from handling the sewing machine by the operating personnel. If, for achieving an increased stability, thicker needles are used, the above-outlined problems become even more pronounced.
During sewing the thread must be protected from being damaged as it is pushed through the fabric by the needle. Therefore, in industrial sewing machines it is a desideratum that the needle, even in case of extremely high reciprocating speeds, ensures a substantially frictionless run of the thread during the entire stitch forming cycle. Since the thread thickness is in most cases predetermined for providing the desired seam strength, the sewing machine needle must guide the thread of predetermined thickness with low friction and must adequately protect the thread even in the high-speed range. Also, an excessive motion caused friction between the thread and the fabric is to be avoided in the high-speed range. In case such a requirement cannot be ensured, uncontrollable thread ruptures and misses in the stitching operation result.
German Offenlegungsschrift (application published without examination) 30 27 534 discloses a sewing machine needle having a laterally bent blade. A thread trough provided along the needle blade leads to a needle eye which is positioned in the vicinity of the needle point. The thread trough is defined by two lateral walls which extend into the eye region. The needle contour is substantially linear in the region of transition from the laterally bent zone to the needle point. The above-described difficulties during high-speed sewing operations may involve sewing machine needles of the above-outlined conventional type.